1. Field of the Invention
This invention relates to improvements of the zoom lens of compact size and light weight, while still maintaining an F-number of 1.6-2.0 or thereabout and a zoom ratio of about 6, suited to video cameras or still cameras.
2. Description of the Related Art
In the so-called 4-component zoom type, the combination of the zoom ratio of about 6 with the use of 6 members in the fourth lens unit which has the image forming function is exemplified by previous proposals in Japanese Patent Publication No. Sho 63-61642, Japanese Laid-Open Patent Application No. Sho 62-91908 and others.
But, in recent years, the image pickup element has its size get smaller, for example, from 1/2 inch to 1/3 inch. To preserve the standard angle of view, therefore, it is necessary to relatively decrease the focal length of the zoom lens. To rely on shortening the focal length of the relay lens unit, however, results in a difficulty of assuring the prescribed value of the back focal distance for disposing the filter. This problem can be solved by reducing the air spacing within the relay lens unit, so that the rear principal point of that relay lens unit falls at a shorter distance. Such a shortening of the spacing between the front and rear lens sub-units of the relay lens unit, as proposed in the Japanese Laid-Open Patent Application No. Sho 62-91908 and others, however, leads to difficult aberrational problems. Particularly in the intermediate angle of view, the coma flare and the curvature of field are caused to increase largely. Thus, an adverse tendency is brought in with an increase of the residual amount of aberrations when it should be further minimized in view of the decreased size of the image frame.
Hence, there is a growing desire for such an optical system that, while making it possible to adequately leave the spacing between the front and rear lens sub-units of the relay lens unit in order to correct aberrations with ease, the back focal distance, too, is secured sufficiently.